Clinical library · Outcome measures
WOMAC
The standard patient-reported measure in hip and knee osteoarthritis. Reliable, widely translated, and carrying two problems worth knowing: its pain and function scales are not really separate, and its published MCIDs span a threefold range.
In one line. Twenty-four items in three subscales — 5 pain, 2 stiffness, 17 physical function — self-reported, describing the previous 48 hours in hip or knee osteoarthritis. Scored on Likert or visual analogue formats, and normalised to 0–100 in most reporting.
It is the most-used disease-specific instrument in osteoarthritis and it does its job. The two things to hold in mind are that its pain subscale behaves like a function subscale, and that no single MCID for it can be quoted honestly.
There is no such thing as "the" WOMAC MCID. A systematic review of 13 studies found MCIDs after total knee replacement ranging from 13.3 to 36.0 for pain and 1.8 to 33.0 for function, and after total hip replacement from 8.3 to 41.0 for pain and 9.7 to 34.0 for function. The reviewers conclude that the variability in methods makes it unclear whether values in the literature can be applied with confidence. [2] Any page that gives you one number for this is not reading its own sources.
The numbers you actually need
| Property | Value | Source and caveat |
|---|---|---|
| Internal consistency | Function consistently 0.90 or above; pain and stiffness 0.70 or above | 76 articles from 22 countries, assessed against COSMIN criteria — which most studies did not fully meet [1] |
| MCID range, total knee replacement | Pain 13.3 to 36.0; function 1.8 to 33.0 | 13 studies, differing anchor and distribution methods and differing baseline scores [2] |
| MCID range, total hip replacement | Pain 8.3 to 41.0; function 9.7 to 34.0 | Same review [2] |
| One worked example, after TKA | MCID 11 pain, 9 function, 10 total; MDC95 23 pain, 11 function, 12 total | Adjusted for age, sex, BMI and baseline scores. Note the MDC for pain (23) exceeds its own MCID (11) [3] |
| One worked example, after high tibial osteotomy | MCID 4.2 pain, 1.9 stiffness, 10.1 function, 16.1 total | Different procedure, different population, different numbers entirely [4] |
| Pain versus function correlation | Median 0.79 | Five exploratory factor analyses did not support pain and function as distinct constructs; the pain scale did not show divergent validity [1] |
| Ceiling effects | 24% to 38% at best possible score after arthroplasty | On pain and stiffness scales, 1 to 23 years post-operatively [1] |
What it measures
Symptoms and function attributable to hip or knee osteoarthritis, from the patient's own report over the previous two days. The function subscale carries 17 of the 24 items, so the total is dominated by function whatever the label on the instrument says.
Normative values now exist. In 714 healthy adults, WOMAC pain, stiffness and function all varied significantly with age and sex, which means a "normal" score is not a fixed target and should be read against the patient's own demographic band. [7]
Where it misleads
1. The pain subscale is not a clean pain measure
This is the most important finding on the page and the least widely known. Five exploratory factor analyses did not support a model in which pain and function items were distinct; the two scales correlated at a median of 0.79; and the pain scale correlated with other pain measures no differently from the function scale, so it failed to demonstrate divergent validity. [1] Reporting "WOMAC pain improved" is close to reporting that WOMAC function improved.
2. Quoting an MCID without its procedure and method is meaningless
Compare the two worked examples in the table. After total knee arthroplasty, one well-adjusted study reported an MCID of 11 for pain and 10 for the total. [3] After medial opening wedge high tibial osteotomy, another reported 4.2 for pain and 16.1 for the total. [4] Both are correct for their populations. Neither transfers.
Worse, in the TKA study the 95% minimal detectable change for pain was 23 — larger than the MCID of 11 for the same subscale. [3] For that subscale, in that population, a change at the MCID cannot be distinguished from measurement error in an individual. The authors note that for the function and total scores the measurement error was smaller than the minimal important change, which is the reassuring half of the same finding.
3. A simple visual analogue scale outperformed it for detecting treatment effects
A meta-epidemiological study of 28 trials and 44 randomised comparisons found that in 64% of comparisons the effect size from a global pain VAS favoured the intervention more than the WOMAC pain subscale did, and in 11 of 12 meta-analyses the VAS showed larger benefit, with a combined difference in effect size of -0.08 (95% CI -0.14 to -0.02) and no relevant increase in between-trial heterogeneity. [5] The elaborate instrument was less sensitive than a single line.
4. Ceiling effects after joint replacement are substantial
Between 24% and 38% of patients achieved the best possible score on the pain and stiffness scales in the years after arthroplasty. [1] Once a quarter to a third of your cohort is at the ceiling, the instrument cannot show further improvement or detect deterioration.
5. In meniscal injury, it was outperformed
In a head-to-head study in patients with meniscal tears, the IKDC showed the best performance across all measurement properties, and the authors conclude the IKDC rather than KOOS or WOMAC should be used in that population. Ceiling effects within the smallest detectable difference of the maximum score were found for all WOMAC dimensions. [6]
What the evidence supports — and what it does not
Supported
- Reliable measurement in hip and knee osteoarthritis, with high internal consistency and acceptable test-retest reliability. [1]
- Equivalence of paper and electronic administration. [1]
- Population-specific MCIDs where they exist, quoted with their procedure and derivation. [3][4]
- Comparison against age- and sex-adjusted normative values. [7]
Not supported
- Treating the pain subscale as a distinct pain measure. No divergent validity. [1]
- Any single MCID. Published values span roughly threefold. [2]
- Interpreting an individual pain change at the MCID after TKA. The MDC is larger. [3]
- Assuming it is the most sensitive available pain endpoint. A global VAS beat it. [5]
- Using it in meniscal injury in preference to the IKDC. [6]
How certain is this?
Evidence grade: Moderate.
The measurement-properties evidence is broad: 76 articles from 22 countries assessed against COSMIN. Its own caveat is that COSMIN criteria were not fully met in most included studies, so the synthesis is of a large but methodologically uneven literature. [1]
The MCID position is the most secure conclusion here, and it is a negative one. The review that establishes the range is explicit that variability in method makes it unclear whether published values can be applied with confidence. [2] The two worked examples are single-centre cohorts.
The assay-sensitivity finding is a meta-epidemiological analysis across 28 trials, which is a strong design for that question, though the absolute difference in effect size (-0.08) is small. [5] The head-to-head meniscal comparison is a single Dutch-language validation study. [6]
What would change the grade: MCID derivation using a standardised method across populations, and a joint-specific pain measure with genuine divergent validity — which the COSMIN review explicitly calls for. [1]
Common questions
What change should I call clinically important?
Whatever the best-matched study for your procedure and population reports, quoted with its source. After total knee arthroplasty one adjusted analysis gave 11 for pain and 10 for the total; [3] after high tibial osteotomy, 4.2 for pain and 16.1 for the total. [4] Across 13 studies the published range for TKR pain alone was 13.3 to 36.0. [2] There is no defensible single number.
Can I report WOMAC pain as a pain outcome?
With care. Five factor analyses did not support pain and function as separate constructs, the two subscales correlate at a median of 0.79, and the pain subscale showed no divergent validity from other pain measures. [1] If pain is your primary endpoint, note that a global visual analogue scale had higher assay sensitivity in 11 of 12 meta-analyses. [5]
Is a 10-point change real in one patient?
Depends on the subscale. In the TKA cohort, the 95% minimal detectable change was 23 for pain, 11 for function, 27 for stiffness and 12 for the total. [3] A 10-point pain change in an individual sits well inside that error; a 12-point total change sits at its edge.
Why do my post-operative patients stop improving on it?
They may have hit the ceiling rather than plateaued. Between 24% and 38% of patients achieved the best possible score on the pain and stiffness scales in the years after arthroplasty. [1] Consider a measure with more headroom for high-functioning patients.
Should I use WOMAC or KOOS for a meniscal tear?
Arguably neither. In a direct comparison in patients with meniscal tears, the IKDC performed best on all measurement properties, and ceiling effects were found for all WOMAC dimensions. [6] See also the section index for the KOOS page, and meniscal tear for the treatment evidence.
References
- Gandek B. Measurement properties of the Western Ontario and McMaster Universities Osteoarthritis Index: a systematic review. Arthritis Care & Research. 2015 Feb;67(2):216–29. doi:10.1002/acr.22415 PMID 25048451 Systematic review (COSMIN)
- MacKay C, Clements N, Wong R, et al. A systematic review of estimates of the minimal clinically important difference and patient acceptable symptom state of the Western Ontario and McMaster Universities Osteoarthritis Index in patients who underwent total hip and total knee replacement. Osteoarthritis and Cartilage. 2019 Oct;27(10):1408–1419. doi:10.1016/j.joca.2019.05.002 PMID 31096046 Systematic review
- Clement ND, Bardgett M, Weir D, et al. What is the Minimum Clinically Important Difference for the WOMAC Index After TKA?. Clinical Orthopaedics and Related Research. 2018 Oct;476(10):2005–2014. doi:10.1097/CORR.0000000000000444 PMID 30179956 Cohort study
- Kim MS, Koh IJ, Choi KY, et al. The Minimal Clinically Important Difference (MCID) for the WOMAC and Factors Related to Achievement of the MCID After Medial Opening Wedge High Tibial Osteotomy for Knee Osteoarthritis. American Journal of Sports Medicine. 2021 Jul;49(9):2406–2415. doi:10.1177/03635465211016853 PMID 34115533 Cohort study
- da Costa BR, Saadat P, Basciani R, et al. Visual Analogue Scale has higher assay sensitivity than WOMAC pain in detecting between-group differences in treatment effects: a meta-epidemiological study. Osteoarthritis and Cartilage. 2021 Mar;29(3):304–312. doi:10.1016/j.joca.2020.10.004 PMID 33271331 Meta-epidemiological study
- van de Graaf VA, Wolterbeek N, Scholtes VA, et al. Reliability and Validity of the IKDC, KOOS, and WOMAC for Patients With Meniscal Injuries. American Journal of Sports Medicine. 2014 Jun;42(6):1408–16. doi:10.1177/0363546514524698 PMID 24618098 Measurement properties study
- Marot V, Murgier J, Carrozzo A, et al. Determination of normal KOOS and WOMAC values in a healthy population. Knee Surgery, Sports Traumatology, Arthroscopy. 2019 Feb;27(2):541–548. doi:10.1007/s00167-018-5153-6 PMID 30251101 Cross-sectional normative study
About this resource
Using this in clinic
Every figure here is traceable to its source.
Every MCID on this page names the procedure, the population and the derivation method, because for this instrument those three things move the number by a factor of three. Where a value could not be verified against the paper it came from, it is not on this page, and the omission is stated rather than filled with a number from a secondary source.